1. Technical Field
The present invention relates to a semiconductor laser element and to a method for manufacturing the same, and more particularly relates to a semiconductor laser element having a conductive oxide layer, and to a method for manufacturing the same.
2. Related Art
Conductive oxide layers have been widely used in the past as electrodes in semiconductor light emitting elements, and particularly in light emitting diodes (LEDs). (See Chia-Sheng CHANG et. al, “InGaN/GaN Light-Emitting Diodes with Rapidly Thermal-Annealed Ni/ITO p-Contacts”, Jpn. J. Appl. Phys. vol. 42 (2003) pp. 3324-3327)
Advantages to a semiconductor laser element are that the conductive oxide layer does not absorb much light, the refractive index is low, and so forth, so a technique has been proposed in which a conductive oxide layer having a refractive index lower than the refractive index of the light emitting layer is formed on a semiconductor layer having a ridge shape, with this conductive oxide layer functioning both as a conductive electrode and a clad layer. (See Patent Literature 1: JP H09-74249-A)
In addition, a technique has been proposed in which a block layer having a stripe-shaped opening is disposed on a semiconductor layer laminate, and a conductive oxide layer is formed on the block layer including this opening, the result being a clad layer electrode equipped with a ridge-shaped protrusion, having the function of both a conductive electrode and a clad layer. (See Patent Literature 2: JP2006-41491-A)
With these semiconductor laser elements, a dielectric layer is usually formed on the side faces of the ridge in order to ensure an effective refractive index distribution in the waveguide region directly below the ridge and the regions directly below both sides of this ridge.
An example of the method for forming the dielectric layer on the side faces of the ridge is a method in which a dielectric layer is formed from the bottom face region of the ridge all the way to a region that reaches the electrode, and etching this dielectric layer using a resist, thereby leaving the dielectric layer on the ridge side faces while removing the dielectric layer from the place where it contacts the electrode.
However, the above-mentioned elements have yet to see practical application because the high resistivity of conductive oxides drives up the voltage, and the electrical characteristics are unsatisfactory. These problems are particularly pronounced when using a current confined path structure because the current density rises in the narrower emission region.
Also, it was difficult with the above-mentioned method for forming a dielectric layer to precisely leave the dielectric layer only on the side faces of the ridge.
In particular, with the semiconductor laser element in Patent Literature 1, the side faces of the conductive oxide layer and the semiconductor layer lie in the same plane, so it is difficult to stop the etching precisely at the boundary between the conductive oxide layer and the semiconductor layer.
Therefore, when there is a region of the ridge side faces that is not covered by the dielectric layer, proper optical confinement becomes impossible, and leakage is caused when the electrode contacts the ridge side face, among other such problems.
Meanwhile, when the dielectric layer is left on the side faces of the conductive oxide layer, the problem with this is that the voltage is raised and the performance of the semiconductor laser element cannot be fully realized.